The present invention relates to a chip fuse and a process for production thereof.
A current protecting element is used for the purpose of protecting electronic devices against overcurrent. It is inserted between a power circuit and an electric circuit. When an overcurrent arises, the wiring housed in the current protecting element is broken and the current is interrupted, and thereby the electric circuit is protected.
As a common name, such an element is called "fuse". An element can be called fuse only when characteristic properties prescribed in various Standards are fulfilled. In view of the current trend aiming at diversification of electronic devices, there is an increasing demand for a current-protecting element of which characteristic properties are different from those prescribed in the prior art fuse Standards.
As a device for preventing overcurrent, not only the above-mentioned current-protecting element but also electronic switches using thyristor or transistor can be used. Such electronic switches, however, are not suitable for use in the fields requiring a small size and a low consumption of power such as cell-worked portable instruments, because the use of such switches brings about an increased number of circuit parts and an increased consumption of electric power.
Thus, JP-A-60-143544 has disclosed an element prepared by forming on a ceramic substrate a silver layer or a silver-palladium layer as a first conductor layer, a nickel layer as a second conductor layer and a solder layer or a tin layer as a third conductor layer in order to improve the clearing characteristics at the time of soldering. Further, this patent gazette has also disclosed a technique of coating the surface of the conductors with a non-combustible or flame-retardant resin such as silicone resin or the like.
However, the product prepared by providing a current-protecting element (fuse) on a ceramic substrate is disadvantageous in that the ceramic substrate is low in heat resistance and, if the current-protecting element is covered with a non-combustible or flame-retardant resin, it exhibits a high heat dissipation so that the current value of clearing tends to vary depending on the environmental temperature.
In order to solve the above-mentioned problem of ceramic substrate, a method of using an organic resin type insulating substrate has been proposed. This method, however, has a problem in that the substrate resin tends to undergo smoking or ignition when the resin is epoxy resin, phenolic resin, polyimide resin or the like.